//Write a program to find the node at which the intersection of two singly linke
//d lists begins. 
//
// For example, the following two linked lists: 
//
//
// begin to intersect at node c1. 
//
// 
//
// Example 1: 
//
//
// 
//Input: intersectVal = 8, listA = [4,1,8,4,5], listB = [5,6,1,8,4,5], skipA = 2
//, skipB = 3
//Output: Reference of the node with value = 8
//Input Explanation: The intersected node's value is 8 (note that this must not 
//be 0 if the two lists intersect). From the head of A, it reads as [4,1,8,4,5]. F
//rom the head of B, it reads as [5,6,1,8,4,5]. There are 2 nodes before the inter
//sected node in A; There are 3 nodes before the intersected node in B. 
//
// 
//
// Example 2: 
//
//
// 
//Input: intersectVal = 2, listA = [1,9,1,2,4], listB = [3,2,4], skipA = 3, skip
//B = 1
//Output: Reference of the node with value = 2
//Input Explanation: The intersected node's value is 2 (note that this must not 
//be 0 if the two lists intersect). From the head of A, it reads as [1,9,1,2,4]. F
//rom the head of B, it reads as [3,2,4]. There are 3 nodes before the intersected
// node in A; There are 1 node before the intersected node in B.
// 
//
// 
//
// Example 3: 
//
//
// 
//Input: intersectVal = 0, listA = [2,6,4], listB = [1,5], skipA = 3, skipB = 2
//Output: null
//Input Explanation: From the head of A, it reads as [2,6,4]. From the head of B
//, it reads as [1,5]. Since the two lists do not intersect, intersectVal must be 
//0, while skipA and skipB can be arbitrary values.
//Explanation: The two lists do not intersect, so return null.
// 
//
// 
//
// Notes: 
//
// 
// If the two linked lists have no intersection at all, return null. 
// The linked lists must retain their original structure after the function retu
//rns. 
// You may assume there are no cycles anywhere in the entire linked structure. 
// Each value on each linked list is in the range [1, 10^9]. 
// Your code should preferably run in O(n) time and use only O(1) memory. 
// 
// Related Topics 链表 
// 👍 906 👎 0


//leetcode submit region begin(Prohibit modification and deletion)
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        // 1.hash
//        if(headA == null || headB == null) {
//            return null;
//        }
//        Set<ListNode> nodes = new HashSet<>();
//        ListNode cur = headA;
//        while(cur != null) {
//            nodes.add(cur);
//            cur = cur.next;
//        }
//        cur = headB;
//        while (cur != null) {
//            if(nodes.contains(cur)) {
//                return cur;
//            }
//            cur = cur.next;
//        }
//        return null;
        // 2.双指针 
        if(headA == null || headB == null) {
            return null;
        }
        ListNode node1 = headA;
        ListNode node2 = headB;
        while(node1 != node2) {
            node1 = node1 == null ? headB : node1.next;
            node2 = node2 == null ? headA : node2.next;
        }
        return node1;

    }
}
//leetcode submit region end(Prohibit modification and deletion)
